Depend on nbclassic 0.4.7

[echarles]

This PR upgrades the nbclassic dependency to 0.4.7.

[bnavigator]

Just wanted to let you know that from a distribution point of view this pinning and bumping out of sync is really annoying.

[echarles]

@bnavigator thx for your feedback. We have today a community call (https://github.com/jupyter/notebook-team-compass) where we can discuss that. You are welcome to join, or share here more context (why, conseqences, alternatives...)

[echarles]

We have discussed that during the community call and the conclusion was that we should define a lower boundary instead of a strict pin. I have just pushed a commit that defines nbclassic>=0.4.7

[bnavigator]

Thank you. I understand that sometimes it is necessary, e.g. the pinning to 0.4.5 because 0.4.6 broke it (#6580) But when you do, please at least add a comment and explain it in your commits/pull requests. For example, #6570 is unnecessarily opaque.

[echarles]

Right, I have added a small explanation on #6570 (comment)

[RRosio]

LGTM, thanks @echarles!